Method and device for the electrolytic treatment of high-resistance layers

ABSTRACT

A method and a device for electroplating and electrolytically etching plate-shaped or strip-shaped material in continuous installations or bath installations having rotating transport and contact structures along the conveyor belt. The current is fed to the material in the center, i.e. in the useful zone, thereby obtaining a layer thickness distribution which is at least as good as when the current is supplied from both edges. The invention further allows material of any formats of width and different contours to be electrolytically treated in any order.

The invention relates to the electrolytic treatment, in particular electroplating and etching, of electrically conductive layers on preferably planar material. It is particularly suitable for electroplating substrates such as printed circuit boards and conductive foils as portions in continuously operating plant or strips of metal or metallized synthetic films in plant for passing material from one roll to the next. Here, the intention is to apply as high a current density as possible to deposit or etch a metal layer of uniform thickness over the entire extent of the surface of the material even if the base layer is very thin and hence has high resistance. Tried-and-tested solutions for this already exist.

It is the object of the invention to describe making electrical contact in planar material for electroplating or for electrolytic etching in continuously operating plant and in plant for treating material which is passed from one roll to the next. In particular, the preferably rotating electrical contact-making means of the material is also to be suitable for material having differing sizes transversely to the direction of transport and having thin base layers, for uniform electrolytic treatment. In this case, by comparison with the prior art a high level of complexity in the plant is to be avoided.

The object is achieved by the method according to claim 1 and the device according to claim 7. The subclaims describe advantageous embodiments of the invention.

Continuously operating plant and plant that produces material passed from one roll to the next is suitable preferably for the manufacture of mass-produced products, because it has little flexibility in respect of the sequence of processes. These products were called end products above. These are usually small and miniature printed circuit boards or conductive foils for, for example, BGAs (ball grid arrays), RFIDs (radio frequency identification units), MP3 players, memory sticks or indeed for relatively large printed circuit boards for, for example, mobile telephones, PCs and the like. On the one hand the invention makes use of the increasing miniaturisation of these electronic end products to optimize the layout of the material, and on the other it supports the precision conductor technology required for miniaturisation, as a result of the planar electroplating of the thin base layers necessary therefor. After they have been finished in the continuously operating plant, and where appropriate further plant, the end products are separated from the material for their respective use.

The material to be treated is in this case for example a large-scale printed circuit board or conductive foil. In practice, these large-scale printed circuit boards or conductive foils are also called panels. They have a useful and a non-useful zone. The end products are located in the area to be used. By contrast, the edge zones, including the contact track or tracks located thereon, are typically not usable for the end products. In the layout of the printed circuit boards, a large or very large number of, usually, the same end products are arranged in the area to be used. There are always a plurality of ways of arranging the many end products on the printed circuit board or panel. The present invention makes use of this fact. It takes as a starting point the fact that the end products are arranged on the panel such that a contact track is formed on the material and kept free approximately or precisely in the centre, transversely as seen in the direction of transport. Similarly approximately or precisely in the centre of the transport track of the continuously operating plant or strip plant, that is to say also transversely as seen in the direction of transport, there is in each case an electrical contact, preferably rotating, on each of the many contact rolls or contact wheels arranged along the transport path, which may equally be the transport rolls, transport wheels or transport means.

The invention is described in particular by way of the example of electroplating, in particular that of printed circuit boards for metalizing the entire surface and providing uninterrupted contact and for constructing the conductor pattern, which is structured using resist, for example. However, the invention is also suitable without restrictions for electrolytic etching and other electrolytic processes.

According to the invention, the material is supplied with the electrical or electrolytic current required for electroplating, preferably by means of a contact track in the centre. In this case, the effect on the distribution of layer thickness transversely as seen in the direction of transport is at least as advantageous as—or better than—in the prior art with optimally uniform supply from the two edges. The two oblique planes which are formed according to the invention face in the opposite direction, however. The maximum layer thickness is once again achieved in the contact zone, that is to say in the centre of the material. The differences in layer thickness in the oblique planes transversely as seen in the direction of transport are, according to the invention, only approximately a quarter of the difference achieved when the electroplating current is supplied from only one edge, regardless of the contact resistances effective at the particular moment.

The making of electrical contact in the region of the centre of the material, that is to say in the useful zone, has the following substantial advantages by comparison with the prior art:

-   -   The method is suitable for different dimensions of the material,         transversely as seen in the direction of transport, without         re-tooling the plant. For this reason, material having different         dimensions and different layouts can be introduced into the         plant and electroplated in any desired sequence.     -   Because the oblique plane is inclined from the centre towards         the two edges, the bone effect at these edges is not disruptive         but in some cases has a useful effect. The layer which is         thinner at the edges is advantageously made thicker by the “bone         formation” laid down by deposition, as a result of which the         difference in the layer thickness from the edge to the centre         becomes even smaller than in the case of supply from both edges,         as in the prior art.     -   The transport and contact rolls require only one contact. This         means that the technical complexity is reduced.     -   The electrolytic current is spread over the material, starting         from the centre track and in all cases reproducibly, to give         parts in size in the direction of the two far edges. For this         reason, the above-mentioned quartering in size of the oblique         plane is always precisely maintained, even in the event of         uncontrollable transfer resistances at the contacts.     -   Only one contact track is required on the material, that is to         say there is less loss of useful zone than when there is supply         from both edges, with as good as or better distribution of layer         thickness.     -   The size of the useful zone can be adapted precisely to the size         of the end products to be arranged thereon, as a result of which         an unnecessarily large amount of waste base material can be         avoided.     -   The current density can be selected individually, depending on         the requirements.     -   Even in the event of sporadically inadequate rotating contact         means demetallisation, it is not possible for there to be         different diameters on the two sides, which would carry the         boards away from the track.     -   Even if, for unpredictable reasons, the boards did leave the         track, the fault described, requiring unscheduled maintenance of         the plant, would not occur. In the event of leaving the track,         electrical contact with the printed circuit board is maintained.         Thus, unreliably intensive metallisation or damage to the         contacts cannot occur. In the worst case, the quality of end         products of the printed circuit board concerned which are         arranged on one side, next to the contact track, would be         impaired.     -   Where very reliable electrical contact is made, damage to the         surface of the material on which the contacts roll can be         avoided. In this case, positioning of the contact track on the         material and its position in relation to the rotating contacts         are in no way critical, because the contacts can go into the         zone of the end products.     -   The width of the contact track can be selected to be smaller         than that in the prior art because the safety margin is not         necessary, which means that the useful zone of the material is         made bigger.     -   In particular in the case of small end products, it is possible         to dispense entirely with a contact track as a separate area on         the material. The contact track extends in the zone of the         centre of the material, as desired, over some of the end         products, which where applicable are later subjected to a         sorting operation and rejected. In this very advantageous         method, the alignment station which in the prior art is always         required upstream of the electroplating plant can be dispensed         with. In particular with thin and hence very flexible material,         alignment stations of this kind are technically very complex. In         this case, according to the invention the material runs through         the continuously operating plant, unaligned in the lateral         direction. Similar unaligned transport of the material is         already performed in wet-chemical treatment stations and rinsing         stations in known manner, both upstream and downstream of the         electroplating plant.     -   The format of the material may differ from the conventional         rectangular shape. For example, it may be triangular, round or         oval.     -   As an extension to the invention, it is also possible to supply         the electroplating current in the useful zone of the material by         means of a plurality of contact tracks which are offset         transversely as seen in the direction of transport. Even when         there are two contact tracks in the useful zone, the local         differences in the cell voltages may be reduced to a sixteenth         of that with a one-sided supply according to the prior art.

The invention will be further described below with reference to the schematic FIGS. 1 to 3, which are not to scale.

FIG. 1 a shows, in cross section, a continuously operating plant or a strip plant, according to the prior art.

FIG. 1 b shows, on a much larger scale, the profile of the layer thickness to be achieved with the arrangement according to FIG. 1 a.

FIG. 2 a shows, in cross section, a continuously operating plant or a strip plant, according to the present invention.

FIG. 2 b shows the profile of the layer thickness to be achieved according to the invention, transversely as seen in the direction of transport, again on a much larger scale.

FIG. 3 shows, in plan view, a material having a large number of end products arranged thereon.

In FIG. 1 a, the material 1 is transported perpendicularly out of the plane of the drawing. For this purpose there serve upper and lower transport and contact means 2 which are driven in rotation. Arranged on the elongate contact means 2 there are, at both ends, annular or disc-shaped electrical contacts 3. These contacts 3 roll on the upper and lower sides of the material 1, on the base layers 4 thereof. During this, the material 1 is transported and at the same time makes electrical contact. The electrical contact is made at the two edge zones 5 of the material. On the upper side and lower side there are soluble or insoluble anodes 6. Together with the respective anodes 6, the base layer 4, that is to say the surface of the material 1, forms a respective electrolytic cell, located in the electrolyte 7. In each case at least one electroplating current source 8, taking the form of a direct current source or a unipolar or bipolar pulsed current source, serves to supply the electrolytic cells with current. The electrical current is transmitted by way of rotary contacts 9 or sliding contacts 9 to the rotating transport and contact means 2 and from there to the contacts 3. The width of the material 1 transversely to the direction of transport must be dimensioned such that the contacts 3 can each roll on a sufficiently wide contact track at the edges of the material 1. For this reason, only material 1 of a very particular width can be handled in an existing plant according to the prior art.

FIG. 1 b shows the profile of the layer thickness on the material 1 transversely to the direction of transport, as results from an arrangement according to FIG. 1 a. The profile of the oblique planes, which are illustrated on a very large scale, is typical. The minimum layer thickness of the electroplated layer 10 occurs in the zone at the centre of the material 1. The effective cell voltage between the cathodic surface of the material 1 and the anode 6 is smallest in this zone, because of the voltage drop in the base layer 4. Accordingly, the local current density and hence the thickness of the deposited layer are also smallest here. In this arrangement, there are consequently the following dependent relationships:

The difference in the amount deposited between the edge zone 5 and the centre of the material increases when the base layer to be electroplated is of higher resistance, when the width of the material transversely to the direction of transport is larger and when a higher current density is used for the electroplating.

The so-called deposition bone formation 11 is added to the profile of the oblique planes at the already higher edges of the material 1, and this effect is particularly intensive there because of the electrical edge effect in the zone of greatest local current density. This increases the difference in the overall depositions on the material in a highly disadvantageous way. In particular for the above-mentioned mass-produced products which are made by precision conductor technology, the distributions of layer thickness have to be very uniform, and according to the prior art this can only be achieved using low current densities.

FIG. 2 a shows, in cross section, a continuously operating plant or a strip plant according to the invention, for electroplating board-shaped or strip-shaped material 1. In this plant, there are along the transport path numerous transport and contact means 2 which transport the material 1 and make electrical contact. Rolls are illustrated serving as the transport means 2. It is also possible for rotating rolls having small wheels, or non-rotating sliding contacts, to be used. The electrical contacts 3, which take the form of rings, small wheels, discs, brushes or segmented contact wheels, are in this basic arrangement of the invention located transversely as seen in the direction of transport, preferably in the centre of the contact means 2 and the transport path. This arrangement requires only one corresponding contact track 15 on the material, preferably also running in the centre of the material 1 inside the useful area 17, as shown in FIG. 3. This means that a respective useful part-zone or a useful part-area 12 of the material is located to either side of the contact track 15. The electroplating current is supplied to the material by way of the contact track 15 on the material, this track being kept free separately or not separately within the layout and running within the entire useful zone thereof. In the case of a contact track 15 which is not kept free and separate this extends over the end products 14 arranged in the layout of the material 1.

An asymmetrical contact track, in the layout of the material or printed circuit board or the strip to be electroplated, and corresponding contacts 3 along the transport path of the continuously operating plant may also be provided.

Because electrical contact is still made in the event of the contacts 3 leaving the track unexpectedly, as a result of the arrangement according to the invention in the useful zone, the serious consequences for the plant technology which are described above do not occur. Because of this, there is no need for a safety margin and the width of the contact track 15 may be narrow in the layout of the material 1, for example 10 mm with a width of the contact wheel 3 of for example 5 mm. The contacts always roll within the useful zone 12 of the material. They cannot fall off the material and so lose electrical contact. This means that electrical contact cannot be broken, which among other things means that faults cannot result in the case of demetallisation of the contacts because of a thick metallized layer that was not planned for.

The distribution of layer thickness which can be achieved according to the invention transversely to the direction of transport is shown in FIG. 2 b. The troughs of the oblique planes are in the zones away from contact, that is to say at the two edges 13 of the material 1. These thinner edge zones 13 are adjoined, in the zone of the lowest current density and hence in the trough of the oblique plane, by the respective deposition bone formation 11. Thus, the bone formation occurs in the zone of lowest current density. Because of this, by comparison with the supply on both sides according to the prior art, in which the deposition bone formation is in the zone of highest current density, here it is significantly smaller. In total, therefore, the overall differences in layer thickness are smaller than with the supply on both sides according to the prior art. Added to this are the further advantages described, of making electrical contact with the material in its centre zone.

The method according to the invention is highly suited for example to the requirements currently made of such electroplating plant in printed circuit board technology. These requirements include copper base layers down to a minimum of 1.5 μm thick for printed circuit boards that are 610 mm wide and to which an electroplated layer up to 25 μm thick is to be applied. In this case, only differences in layer thickness of at most 1 μm are acceptable in the zone of the useful area. These requirements can be met according to the invention.

In the case of base layers applied by sputtering or chemically deposited copper layers having a thickness of, for example, 0.2 μm, it is helpful to extend the concept of the invention because of the substantially higher resistance. For this, it is proposed that at least two contact tracks be provided in the layout of the material and two contact tracks or contacts 3 be provided on the contact means 2. These two contact tracks are located inside the useful zone of the material, approximately at ¼ and ¾ of the way across the width thereof, transversely as seen in the direction of transport and approximately symmetrically in relation to the transport path. Here, a total of four smaller oblique planes are formed on the upper side and where appropriate on the lower side of the material. In this case, ¼ of the total current of this contact zone flows from each contact in each of the two directions, transversely as seen in the direction of transport. At the same time, the length of the current flow in the base layer of the printed circuit board is reduced to ¼ of the total width, resulting in the electrical resistance of the associated current path being quartered in size. A quarter of the current flowing through a quarter of the resistance gives, by Ohm's law, a drop in electrical voltage to only one sixteenth. The differences in layer thickness on the material are reduced to approximately this fraction by comparison with the supply of current from one side, according to the prior art. Once again, the troughs of the oblique planes are in each case away from the contacts.

Base layers deposited by sputtering are particularly thin in the edge zone of the material, or metallisation is completely absent. The same is true of so-called etched-back full-surface printed circuit boards. In these, base layers which are for example 12 μm or 17 μm thick are etched back to around 3 μm. Then the actual treatment of the printed circuit boards takes place. In particular because of the puddling effect, the edge zones are etched more intensively than the centre zone. In both these cases, the fact of supplying the electrolytic current in the centre of the material, according to the invention, or in a plurality of tracks of the centre zone proves very advantageous, because that is where the nominal layer thickness for the base layer always prevails and so a reliable supply of current is possible. A plurality of contact wheels arranged transversely as seen in the direction of transport are preferably arranged symmetrically in relation to the transport path. The edge of the material is not required for making contact when supply is in the centre, by means of one or more contact tracks 15. However, in these cases the layout of the material and the position of the contacts on the contact means must be adjusted to one another. If there are a plurality of tracks arranged transversely as seen in the direction of transport, there is no longer complete freedom in the selection of parameters, as is provided by a single supply in the centre. With some mass-produced products which are manufactured over a long period, this extension according to the invention is very advantageous, particularly since there is no economic alternative of comparable simplicity for the electroplating of very thin and hence high-resistance base layers on a large material using high current density, that is to say economic electroplating with a very good distribution of layer thickness.

If there are, for example, two contact tracks 15, it is possible for there to be two contacts 3 on one contact means 2. For this, two sliding or rotary contacts 9 are required if there are associated with each contact track one or more individual rectifiers, which advantageously ensure that there is a current flow of exactly the same size on all sides, transversely as seen in the direction of transport. However, it is also possible to equip each contact means 2 with only one contact 3 and one rotary contact 9. In this case, these contacts 3 are arranged alternately to right and left on the contact means 2, as seen in the direction of transport of the material 1. With this lower-cost solution, the contacts 3 then have to transmit twice the current, however, regardless of whether both sides are supplied by one rectifier or by individual rectifiers.

In particular in the case of high current densities, the size of the current to each contact also increases. In this case, it is important that the electrolytic handling current of a common rectifier is distributed uniformly over all the respectively involved contacts to avoid damage to the surface of the material and/or the contacts. This is particularly important when the contact track(s) in the useful zone run over the end products. As the number of contacts associated with a rectifier 8 decreases, so does the possibility of overloading individual contacts. The best case is when there is a rectifier 8 associated with each individual contact. The current-regulated rectifier limits the treatment current to the pre-set current. This reliably prevents the contact from being overloaded.

FIG. 3 shows, in plan view and by way of example, a material 1 whereof the layout is constructed for electroplating a large number of end products 14 in a continuously operating plant or strip plant according to the invention. The external dimensions of the material are for example 610 mm×610 mm. The useful zone 17, which is edged with a dashed double line and on which the end products and the at least one contact track 15 are located, is smaller by the amount of the narrow edge zones 13. The transport direction arrow 16 indicates the direction of transport of the material through the electroplating plant. The material may also include through holes for electroplating and blind holes that are provided with an electrically conductive layer.

The useful zone for the end products 14, for example BGAs, is smaller, for example 580 mm×580 mm. The edges 13 are not usable for end products 14. In the centre of the material 1, or approximately in the centre, runs the contact track 15, which is kept free and separate. This track is typically not usable for end products 14, or only to a limited extent. In particular in the case of small end products 14, however, it is also possible to cover the entire useful area in the layout with end products and not to leave any contact track on the material as a separate area. Electrical contact is made, in the zone of the centre of the material 1, with the end products 14 there. If these relatively few end products become faulty because of the contact made, they can be discarded in a sorting operation later, once they have been depanelled. This procedure makes the construction of the layout of the material 1 simpler. Moreover, the course of the actual contact track on the material during electroplating is then in no way critical. With small end products, the yield per panel is about the same in both cases, that is to say with or without the contact track as a separate area. In this case, there is no need either for a technically complex alignment station upstream of the continuously operating plant, which is otherwise required for precise lateral alignment of the material.

The invention is also suitable for electroplating structures which are formed by a structured resist on the material. In this case, the contact track, like the other areas to be electroplated, has to be kept free of resist.

The format of the material according to the invention is not restricted to a rectangular shape. It is possible for example for it to have polygonal or round contours. In particular cases, this may result in a saving of base material.

LIST OF REFERENCE MATERIALS

1 Material, printed circuit board, panel

2 Contact means, transport means

3 Contact, contact ring, contact wheel, contact roll

4 Base layer

5 Edge zone

6 Anode, electrode

7 Electrolyte

8 Electrolytic current source, rectifier

9 Rotary contact, sliding contact

10 Electroplated layer, etched layer

11 Deposition bone formation

12 Useful part-area, useful part-zone

13 Edge, edge area

14 End product

15 Contact track

16 Arrow for direction of transport

17 Useful area 

1. A method for the electroplating or electrolytic etching of plate-shaped or strip-shaped material (1) in continuously operating plant or strip plant for passing material from one roll to the next, having transport and contact means (2) along a transport path, characterised in that the material is supplied with the electrolytic current by means of contacts (3) on the transport and contact means (2) by way of at least one contact track (15) which runs inside the useful area (17) of the material (1).
 2. A method according to claim 1, characterised in that electrical contact is made and the electrolytic current is supplied to the material transversely to the direction of transport, in the centre zone thereof.
 3. A method according to either of claims 1 and 2, characterised in that material (1) having different contours and/or different dimensions is brought into electrical contact in any desired sequence in the continuously operating plant as a result of the electrical contact made by means of at least one contact track (15) inside the useful area (17).
 4. A method according to one of claims 1 to 3, characterised in that with structural electroplating or structural etching the electrolytic current is supplied by way of at least one resist-free contact track (15).
 5. A method according to one of claims 1 to 4, characterised in that the electrolytic current is supplied by way of at least one contact track (15) which is kept free within the layout of end products (14) on the material or by way of at least one contact track (15) which extends over the end products (14) in the layout of the material.
 6. A method according to one of claims 1 to 5, characterised in that an individual rectifier (8) associated with each contact (3) supplies it with electrolytic current, or a common rectifier (8) supplies a plurality of contacts (3) with electrolytic current.
 7. A device for the electroplating or electrolytic etching of plate-shaped or strip-shaped material in continuously operating plant or strip plant, having transport and contact means (2) along the transport path, using the method according to claim 1, characterised in that, transversely as seen in the direction of transport, the position of the at least one contact (3) on each transport and contact means (2) and the at least one corresponding contact track (15) on the material (1) are located inside the useful area (17) of the material (1).
 8. A device according to claim 7, characterised in that, transversely as seen in the direction of transport, the position of the contact (3) on each transport and contact means (2) and the corresponding contact track (15) on the material (1) are located approximately or precisely in the centre of the transport path.
 9. A device according to either of claims 7 and 8, characterised in that the usable zone of the material (1) for the end products (14) is also located in the zone of the contact track(s) (15). 